1. Field of the Invention
This invention relates in general to magnetic transducers for reading information signals from a magnetic medium and, in particular, to an improved magnetoresistive read transducer.
2. Description of the Prior Art
The prior art discloses a magnetic transducer referred to as a magnetoresistive (MR) sensor or head which has been shown to be capable of reading data from a magnetic surface at great linear densities. An MR sensor detects magnetic field signals through the resistance changes of a read element made from a magnetoresistive material as a function of the amount and direction of magnetic flux being sensed by the element.
The prior art also teaches that in order for an MR element to operate optimally, two bias fields should be provided. In order to bias the material so that its response to a flux field is linear, a transverse bias field is generally provided. This bias field is normal to the plane of the magnetic media and parallel to the surface of the planar MR element. Commonly assigned U.S. Pat. No. 3,840,898 describes an MR sensor in which transverse bias is produced of a level sufficient to bias the head to the most linear range of the R-H characteristic curve. The bias is produced by a hard magnetic bias layer which is separated from the MR layer by an insulating layer, and the layer of permanent magnet material extends over the entire MR sensor.
The other bias field which is usually employed with MR elements is referred to in the art as the longitudinal bias field which extends parallel to the surface of the magnetic media and parallel to the lengthwise direction of the MR element. The function of the longitudinal bias field is to suppress Barkhausen noise which originates from multi-domain activities in MR elements
Numerous prior art biasing methods and apparatus for MR sensors have been developed However, the drive toward increased recording density has led to the requirement for narrower recording tracks and increased linear recording density along the tracks. The small MR sensors which are necessary to meet these requirements cannot be made with the use of the prior art techniques.
The conceptual solution to these prior art problems was attained only recently through the implementation of patterned longitudinal bias. This solution is described and claimed in the commonly assigned U.S. Pat. No. 4,663,685. Briefly, this invention advocates the creating of appropriate single domain states directly in the end regions only of the MR layer. This can be achieved by producing a longitudinal bias in the end regions only of the MR layer to maintain the end regions in a single domain state and these single domain states induce a single domain state in the central region of the MR layer. In a specific embodiment of this concept, the longitudinal bias is provided by means of exchange coupling between an antiferromagnetic material and a soft magnetic material.
U.S. Pat. No. 4,639,806 describes an MR sensor in which longitudinal bias is provided by layers of ferromagnetic material, which are in contact with the MR element in the end regions coextensive with the electrical leads, and provide longitudinal bias by means of exchange coupling between the layers of ferromagnetic material and the MR element so that the MR element is magnetized in a particular direction along the MR element However, this may have a problem with the permanence of bias since the intrinsic coercivity of the hard-bias film is substantially quenched when exchange coupled to the soft magnetic MR sensor. A second problem is caused by the magnetic flux from the hard-bias film since the added flux causes a longitudinal bias which adversely affects the transverse sensitivity profile and may therefore limit narrow-track extendibility of this approach.
No prior art is known in which a hard-bias film is coupled magnetostatically to an MR sensor in the end regions only.